1. Field of the Invention
The present invention generally relates to piezoelectric crystal devices, and more particularly to edge-mounting configurations for AT-strip resonators used in numerous types of electronic equipment.
2. Description of the Prior Art
Piezoelectric resonators, such as AT-cut quartz crystals, are particularly well adapted for use in a wide variety of crystal filter and oscillator applications for communications equipment. With advancements in electronics miniaturization, a corresponding need arises for a tiny, reliable, inexpensive crystal resonator that retains its stringent electrical performance characteristics. Conventional approaches include the tuning fork-type crystal resonator and the disk-type crystal resonator. The tuning fork-type resonator, typically of XY'-cut quartz operating in the flexure vibration mode, is limited to low frequency applications of less than one MHz., i.e., crystal oscillators for wristwatches. The disk-type crystal resonator, typically made from AT-cut quartz operating in thickness-shear mode, accommodates these high frequency applications--but it is of such large dimensions that its use is severely restricted in many microelectronic applications. When the conventional disk-type crystal device is reduced in size, it exhibits substantial degradation in performance; particularly, temperature characteristics, motional capacitance, and resonator Q.
Recently, the AT-strip resonator was introduced as an improvement to the disk-type resonator to fulfill the aforementioned size, cost, and reliability objectives. The AT-strip resonator, comprised of a thin, rectangularly-shaped crystal blank, is of extremely small size (on the order of 240 mils.times.70 mils.times.4 mils), has an inherently low product cost (due to its minimal quartz usage and simple construction), and achieves improved mechanical integrity through shock and vibration (due to the small mass of the crystal blank).
The prior art literature describes two basic methods for mounting rectangular strip resonators: the cantilever mount and the parallel-plane mount. In the cantilever mounting configuration, the crystal blank is supported at a single end in an upright orientation such that the rectangular blank rests on its short edge. Mechanical support is typically provided by non-conductive cement applied to the short edge of the blank, and electrical connections are provided by a second application of conductive cement along the same edge to contact the electrodes. Although this AT-strip mounting configuration generally exhibits an acceptable level of electrical performance, it provides less than optimal mechanical shock and vibration performance in many applications specifying a particular type of crystal package. Furthermore, the cantilever mounting configuration also requires the use of a mounting fixture to fix the crystal in the upright position during the curing time of the cement.
In parallel-plane mounting, both short ends of the rectangular crystal blank are supported--either by affixing mechanical clips to the short edges of the rectangular crystal blank, or by cementing both short edges to the base--such that the surface plane of the crystal blank is parallel to the major surface plane of the base. Although the parallel-plane mounting method yields devices which demonstrate acceptable shock performance, the electrical performance of the resonator typically degrades from that of the cantilever-mount configuration. In the cement-mounted example, one reason for this degradation is that a portion of the uncured cement can migrate along the crystal surface and contaminates the active region of the resonator, thus degrading the crystal's motional resistance parameter. If the plane of the resonator is elevated above that of the base to inhibit this migration, then taller mounting bases and covers are required. Furthermore, the use of conductive cement for electrical connections to a parallel-plane-mounted resonator necessitates a second cement cure cycle for the cement applied to the top surface electrode.
A need, therefore, exists for an improved mounting configuration for rectangular AT-strip resonators which addresses both electrical resistance parameters and mechanical shock performance, as well as manufacturability.